In process and automation technology, it is known to ascertain or monitor the fill level of a medium by means of the capacitive measuring method. In such method, a probe unit and the wall of the container, or the probe unit and a second probe unit, form, together with the medium as dielectric, a capacitor. The capacitance of this capacitor is measured, and, on the basis of the measured value, fill level is ascertained. Problematic with this method is that the probe unit comes in contact with the medium and that, therefore, accretions can build on the probe unit. Such accretion leads to degradation of the measurement or even prevents measurement. In the state of the art, it is known to supply the probe unit with a relatively high measuring frequency (e.g. greater than 1 MHz), in order to improve accretion tolerance. Disadvantageous in the case of a high measuring frequency is that such is associated with a decrease in the maximum allowable probe length. This is because frequency-dependent resonance effects arise on the probe, which prevent linear measurement.
In the case of accretion formation between container wall and probe unit, a further possibility is to use so-called guard electrodes, which lie at the same electrical potential as the probe electrode and surround the probe electrode (see e.g. DE 32 12 434 C2). Depending on the character of the accretion, it is, however, possible that there will be difficulties with suitable production of the guard signal (DE 10 2004 050 495 A1).
In this case, the probe unit is thus composed of a probe electrode and an auxiliary electrode, with the probe electrode serving for the actual measurement and the auxiliary electrode serving as guard.
In the modern measurements technology, it is evermore required that prospective self tests of a measuring device be performed. In this way, for example, early warnings are possible, before the measuring device fails or before the measurement results could become unreliable.